I've been thinking about you problem a little bit ....
I think the check-number would be best stored in a string:

1.)
to get the year you can use the function
string_1 = Year(Now)

2.)
I would store the current number in a table or file
like, if the last check was 2000-0023, the number stored would be 0023

3.)
when adding a new check
get the stored number and add 1
new_number=stored_number + 1

4.)
update the stored number in your table/file
... set it to 24

5.)
add it together and store it in you database
checknumber= string_1 & "-" & new_number


It shouldn't be a problem to make a little tool with which you can increase the stored number